Extreme-ultraviolet lithography (EUVL) involves employing an extreme-ultraviolet (EUV) radiation source that generates EUV radiation having a wavelength that is typically 13.5 nm+/−2%. The EUV radiation is directed to a reflective patterned mask to transfer the pattern onto a photoresist layer supported by a silicon wafer. The use of the small wavelengths associated with EUV radiation allows for the minimum feature size of the imaged pattern to also be small, i.e., as small as 15 nm and below.
Some EUV radiation sources involve the use of one or more lasers that direct respective laser beams to a fuel target to produce a hot plasma that generates the EUV radiation from an EUV emission location. The EUV radiation is collected by one or more collector mirrors and is then directed to an intermediate focus.
Unfortunately, the reaction that generates the EUV radiation also generates debris particles (e.g., ions, atoms and clusters of atoms) that can deposit on and into the surfaces of the one or more collector mirrors. The deposited debris particles adversely affect the mirror reflectance and thus reduce the performance of the EUV radiation source. This contamination of the collector mirrors can happen very rapidly (on the order of seconds) and can reduce the reflectivity of the collector-mirror surfaces to the point where the amount of EUV radiation available to a downstream illuminator is insufficient to perform the EUV exposure process.
To reduce the adverse effects of the collector contamination from the generated debris, it is known in the art to employ a debris-mitigation device (hereinafter, DMD). One type of DMD employs rotating vanes that intercept the debris particles as they travel toward the collector-mirror surface(s). Because the EUV radiation travels at the speed of light, the rotating vanes appear stationary for the purposes of transmitting the EUV radiation, during the transit time of the EUV radiation passing through the DMD. Thus, the reduction in transmission of the EUV radiation due to the vanes is a function of the cross-sectional area the vanes present to the EUV radiation. The reduction in the amount of contamination by the debris particles—which travel many orders of magnitude slower than the speed of light—is a function of the speed of the rotating vanes, their axial extent, the energy (speed) of the debris particles, and the architecture of the DMD, e.g., there may be rotating vanes followed by stationary vanes so that debris that does not “stick” to the vanes on the first encounter will be deflected and have an opportunity to “stick” in a subsequent encounter with a vane downstream.